1. Field of the Invention
The present invention relates to a method for calibrating luminance of a display, a driving circuit for the display employing the method and portable electronic devices and more particularly to the method for calibrating the display made up of light emitting devices, the driving circuit for the display employing the method and the portable electronic devices being equipped with the driving circuit for the display.
The present application claims priority of Japanese Patent Application No.2001-266432 filed on Sep. 3, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
FIG. 7 is a block diagram showing an example of configurations of a conventional driving circuit of a display. FIG. 8 is a perspective view showing an appearance of a portable cellular phone being equipped with the conventional driving circuit of the display.
The conventional driving circuit of the display includes an organic electroluminescence (EL) display device 1, an organic EL display power source 2, a power source 3, an optical sensor 4, an analog/digital (A/D) converting section 5, a voltage controlling section 6, a key inputting device 7, and a memory 8.
The organic EL display device 1 is constructed of an organic EL device using an organic material such as a derivative of stilbene or a like and, as shown in FIG. 8, is mounted on a surface of an approximate central portion inside an upper portion 11a of a housing 11 of the portable cellular phone. The organic EL display power source 2 is made up of a DC/DC converter or a like and, based on a specified supply voltage being fed from the power source 3, produces a high voltage to drive the organic EL display device 1 and feeds the organic EL display device 1. The power source 3 is made up of a battery, a dry cell, or a like and feeds a specified supply voltage to each component of the portable cellular phone. The optical sensor 4 is made up of a solar cell or a like and, as shown in FIG. 8, is mounted on a surface in a vicinity of a portion existing left under the organic EL display device 1 inside the upper portion 11a of the housing 11 of the portable cellular phone. The optical sensor 4 measures an amount of light being substantially proportional to that of light incident on the organic EL display device 1 and outputs an analog light amount voltage VL corresponding to the amount of light.
The A/D converting section 5 converts the analog light amount voltage VL into digital light amount data DL. The voltage controlling section 6, based on the digital light amount data DL, controls the organic EL display power source 2. The key inputting device 7, as shown in FIG. 8, is mounted inside a lower portion 11b of the housing 11 of the portable cellular phone and is made up of a ten-key and a variety of buttons. The ten-key is used for inputting a telephone number of a person receiving a telephone call. Each of the various buttons is used to issue an instruction for permission and termination of a telephone conversation, switching-over of display, and amendment of current date and calibration of the organic EL display device 1. The memory 8 is made up of semiconductor memories such as RAM, ROM, or a like in which data (correspondence table or converting expression) required for the voltage controlling section 6 to control the organic EL display power source 2 is stored in advance.
Next, operations of calibrating luminance of the organic EL display device 1 based on an amount light incident from an outside in the driving circuit of the organic EL display device 1 having the above configurations are described. The organic EL display device 1 has a characteristic that its luminance changes approximately in proportion to a change of an applied voltage. Moreover, generally, luminance required for a user of the portable cellular phone to recognize contents displayed in the organic EL display device 1 is changed in proportion to a change in an amount of light incident from the outside. That is, in strong light from the outside, unless luminance of the organic EL display device 1 is sufficiently raised, the user cannot recognize displayed contents, however, in the outdoors during the night or in a dimly lit room, even if luminance of the organic EL display device 1 is lowered, the user can sufficiently recognize the displayed contents.
In the above example, the optical sensor 4 measures an amount of light substantially in proportion to light incident on the organic EL display device 1 and outputs the analog light amount voltage VL corresponding to an amount of the light. The A/D converting section 5 converts analog light amount voltage VL into digital light amount data DL. As a result, the voltage controlling section 6, based on digital light amount data DL and on data being stored in the memory 8, controls the organic EL display power source 2 so that a voltage being as low as possible to reduce power consumption is fed to the organic EL display device 1. Thus, according to the example, luminance of the organic EL display device 1 can be calibrated to its minimum level which enables power consumption to be reduced.
In the conventional driving circuit of the organic EL display device 1, the voltage controlling section 6 is required to periodically control the organic EL display power source 2. If a CPU (Central Processing Unit) adapted to control each component making up a portable cellular phone has to control a function of the voltage controlling section 6, there is a problem in that a load put on the CPU becomes large. If the voltage controlling section 6 is mounted independently from the CPU, the portable cellular phone becomes high-priced.
Moreover, in the conventional driving circuit for the organic EL display device 1, in order for the voltage controlling section 6 to control the organic EL display power source 2, the A/D converting section 5 converts the analog light amount voltage VL corresponding to an amount of light to the digital light amount data DL corresponding to the amount of the light and the memory 8 stores, in advance, data required for the voltage controlling section 6 to control the organic EL display power source 2. Therefore, the conventional driving circuit for the organic EL display device 1 has a problem in that, since both the A/D converting section 5 and the memory 8 have to be mounted therein, circuit configurations of the portable cellular phone become complicated and high-priced. The above problems occur also in other portable electronic devices using a battery or a dry cell as a power source such as notebook, palm-sized, and pocket-sized computers, PDA (Personal Digital Assistant), PHS (Personal Handy-phone System) or a like.